The present invention relates to a linear motor.
A variety of types of linear motors are available. Most linear motors that are not required to provide high driving power as in the case of the driving source for a linear motorcar are implemented by combining permanent magnets and electromagnetic coils. A linear motor combining permanent magnets and electromagnetic coils is considered to be applicable to a driving source for a precision micro stage or a precision positioning stage in the field of, for example, semiconductor manufacturing equipment. This is because a driving mechanism based on a linear motor is advantageous in that it provides a higher driving speed and higher positioning accuracy than those provided by a ball screw driving mechanism, which has conventionally been mainstream. The driving mechanism using a linear motor is also advantageous in that it has high repetitious positioning accuracy, has less overshoot and undershoot at starts and stops, and has less speed ripples during constant speed travels.
Referring to FIG. 1, a linear motor that combines permanent magnets and electromagnetic coils will be briefly explained. In FIG. 1, a plurality of permanent magnets 102 are disposed with intervals provided among them at opposing positions of inner walls of a yoke 101 having a substantially U-shaped cross section. A movable coil assembly 103 is provided between the opposing inner walls of the yoke 101 such that the movable coil assembly 103 moves in the direction in which the yoke 101 extends. The movable coil assembly 103 produces a driving force due to the interaction between the magnetic flux generated by the movable coil assembly 103 and the adjoining magnetic flux from the permanent magnets 102. The driving force moves the movable coil assembly 103 by being guided by a guiding mechanism (not shown). The movable coil assembly 103 is normally combined with a member being conveyed, such as a table or stage on which semiconductor wafers are mounted.
When a three-phase drive linear motor is used, the movable coil assembly 103 is provided with a combination of three coils, a U-phase coil, a V-phase coil, and a W-phase coil, as a basic element.
As described above, a conventional linear motor usually combines fixed permanent magnets and a movable coil. Such a linear motor poses the problems discussed below.
First, the linear motor requires a power cable for supplying electric power to the movable coil assembly 103. Since the movable coil assembly 103 is movable, the power cable has to be made flexible to smoothly follow the travel of the movable coil assembly 103. The flexible power cable and a flexible cable supporting member require periodical maintenance or replacement to prevent disconnection. Furthermore, the flexible cable supporting member requires an installation space.
Second, the movable coil assembly 103 generates considerable heat, whereas it is difficult to cool the movable coil assembly 103 since it moves. In addition, a large space for cooling and a complicated cooling structure would be necessary. Unless the problem of the heat generated in the movable coil assembly 103 is solved, the temperature around the movable coil assembly 103 rises. This causes the workpieces being carried, such as semiconductor wafers, mounted on the table to develop deformation, including warping or expansion, making it difficult to maintain dimensional accuracy of the workpieces being carried.
Third, less effective magnetic flux for generating thrust because of the following reasons. In the type of a linear motor shown in FIG. 1, the coil of the movable coil assembly 103 has cylindrical or rectangular coils, while the permanent magnets are merely provided on two opposing surfaces. Increasing the exciting current of the movable coil assembly 103 in order to increase the thrust would make it even more difficult to solve the second problem.
A fourth problem is marked especially in a plane opposing type (flat bed type) linear motor. In this type of linear motor, a plurality of planar permanent magnets are disposed on a stator, while a plurality of coils is installed on a yoke of a moving unit to oppose the permanent magnets. At this time, the permanent magnets and the coils are discontinuously arranged. This causes “cogging” in which the positions where the yoke and the permanent magnets attract each other and the positions where no attraction between them occurs are encountered regularly and successively whenever the moving unit moves in the axial direction. The cogging is considered to directly cause so-called irregular speed.